1. Field of the Invention
The invention relates generally to a method and apparatus of determining gas phase species concentration, and more particularly, to a method and apparatus for detecting concentration of NH-containing species.
2. Description of the Background Art
The use of ammonia (NH3), a corrosive and toxic gas, in industrial processes is wide spread. Trace amount of NH3 has also been shown to adversely impact the use of chemically activated deep ultraviolet photoresists in advanced semiconductor fabrication. The need for worker protection, from either acute exposure to high NH3 concentrations or long term exposure to very low concentration levels, has resulted in the development of sampling methods for the detection and quantitative measurement of NH3 in ambient air. Some existing analytical techniques for NH3 detection are briefly described below.
In this method, gaseous NH3 is absorbed into an electrochemical sensor assembly with a resultant change in the electrical conductivity of the sensor cell. The increased current flow allowed by the sensor is fairly linear over the concentration range of 1-50 ppm. A lower detection limit is about 500 ppb, but reproducibility of the sensor to periodic exposure of NH3 is only fair.
This method uses a reaction between ozone (O3) and ammonia, in which NH3 is first converted to NO2, followed by a chemiluminescent reaction between NO2 and O3. The reaction with O3 results in the formation of excited state NO2 molecules, denoted as NO2*, and the intensity of emission from NO2* is used to determine the original NH3 concentration. However, difficulties in quantitative measurement result from side reactions during the conversion from NH3 to oxides of nitrogen (forming NO and, perhaps, NO3 or HNO), and also from non-stoichiometric side reactions between NO2 and O3. In addition, the emission from excited NO2 species (NO2*xe2x80x94the asterisk xe2x80x9c*xe2x80x9d is used in this disclosure to designate an excited state of a species) extends from the near UV into the yellow-green region of the visible spectrum (this emission is the well known xe2x80x9cair afterglowxe2x80x9d in the night sky, and results from the reaction: NO+O2xe2x86x92NO2*+O). Detection of this very diffuse emission over a broad spectral region is susceptible to interference from other emitting species, and may pose difficulties in accurate concentration determination.
In this method, air samples are collected via a carefully prepared evacuated sampling ampoule and injected into a gas chromatograph (GC) for comparison against analyzed standards by well known methods. Careful selection of the GC column and temperature settings are necessary in order to obtain reliable results. A number of detectors are available for this method. One very sensitive detection method is mass spectrometry, but calibration for quantitative work is very difficult. Additionally, the GC/MS method is very expensive, and it is difficult to configure in a continuous sampling mode.
This method has the potential for great sensitivity, but requires great expertise and expense due to its sophistication. NH3, or a fragment thereof, is electronically (or vibrationally) excited by a pulsed, tunable dye laser, thereby creating observable fluorescence. However, non-linear optical effects and saturation effects tend to make quantitative measurements extremely complex, if at all possible.
Each of these prior art techniques has its own limitation and varying degrees of experimental complexities. Therefore, a need exists in the art for alternative analytical methods which allow continuous on-line determination of low level of ammonia in ambient air or gas samples.
Embodiments of the invention generally provides a method and apparatus for determining the concentration of an NH-containing species in a gas sample. The method comprises detecting radiation from excited imidogen radicals (NH*) generated from the gas sample, and determining the concentration of the NH-containing species from calibration data correlating detected NH* radiation intensity with concentration of the NH-containing species. In one embodiment, the NH-containing species is ammonia (NH3), and the NH* radiation is generated by reacting NH3 with a gas sample containing fluorine. Using a bandpass optical device, NH* radiation around 336 nm can be selectively transmitted and detected, with minimal interference from other emitting species.